Detection of low concentrations of molecular or material impurities is needed for many scientific and industrial applications. Vibrational spectral techniques of Raman scattering and mid-infrared (MIR) absorption, as examples, are widely used for these purposes. Vibrational spectroscopy uniquely identifies molecules by recording intrinsic vibrational spectral features of the molecules. MIR imaging exhibits poor spatial resolution due to the long wavelengths, and presents challenges due to pervasive absorption and the poor quality of mid-infrared light sources and detectors. Although Raman microscopy uses visible or near infrared light, permitting high spatial resolution imaging, Raman interactions are weak, which limits Raman spectroscopic imaging to relatively high concentration levels. Doppler Raman spectroscopy is based on the measurement of small frequency shifts imparted to pulses in a short laser pulse train interacting with coherently excited Raman-active vibrational modes. That is, a short laser pump pulse is directed into a specimen containing molecules having Raman-active vibrational modes. This pulse is followed by a probe pulse that acquires a shift in the centroid of the pulse spectrum due to the coherently excited vibrations. Typical frequency shifts are tens of GHz; however detection of nanomole concentrations of molecules requires resolving shifts of about 500 Hz.
Other examples where measurement of small frequency shifts is required include, but are not limited to, other inelastic scattering processes, measurement of Doppler shifts, or spectrally-dependent absorption processes that reshape the electromagnetic pulse spectrum.
An electromagnetic pulse, such as an optical pulse, is composed of a broad set of frequencies, referred to as the optical spectrum. One characteristic of such a pulse spectrum is the center or centroid of the spectrum, which is generally the object of measurement. The most common method for measuring optical spectra is an optical spectrometer. The center wavelength (or frequency) can be determined by computing the centroid of the spectral distribution. Changes in the spectrum can be detected by re-measuring the spectrum and calculating the centroid of the modified spectrum.
The highest resolution spectrometers have a maximum resolution in the range of 1 GHz, which is orders of magnitude too coarse for the measurements we wish to make. As stated, many applications require detection of much smaller frequency shifts or changes in center wavelength. The target frequency shift might derive from a translation of the spectrum to higher or lower wavelengths, or could be due to a change in the centroid of the power spectrum through some other mechanism that reshapes the spectrum of the light used to probe a physical behavior or interaction.